Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only curative therapy for many advanced hematologic malignancies. Allo-HSCT is associated with significant morbidities and mortality, mainly because of the graft-versus-host disease (GVHD) caused by donor T cells recognizing antigens present in recipient tissues, and initiating an alloimmune response resulting in damage to many organs including the skin, GI tract, and liver in recipient. More than half of all allo-HSCT recipients develop different degrees of GVHD (grade I-IV). About 20% of recipients develop severe GVHD (grade III-IV) that is associated with a very high morbidity and mortality. The most important factor determining the severity of GVHD is the genetic disparities between donors and recipients, as is reflected in HLA haplotypes that are routinely checked for donor selection. But even when donors and recipients are HLA-identical many recipients develop severe GVHD. As a result, non-HLA antigens are important determinants of acute GVHD. An interesting and unique aspect of GVHD is that it is the consequence of an interaction between antigens present in one individual with the immune system of another individual. As a result, genotypes of both donor and recipient, and their interactions affect the pathogenesis of GVHD. To determine the genetic risk factors for GVHD, in the first aim (discovery phase), we will conduct Genome-Wide Association Studies (GWAS) in 3,000 patients who underwent allo-HSCT and in their respective donors (6000 DNA samples) that are provided to us by the National Marrow Donor Program (NMDP) and the Center for International Blood and Marrow Transplant Research (CIBMTR). This part of our grant is approved by the Center for Inherited Disease Research (CIDR) and will be conducted in their sequencing facility. We expect that 3000 recipients will be stratified into 1200 subjects with moderate to severe acute GVHD (grades II-IV) versus 1800 individuals with none to mild GVHD (grade 0-I). We will also include the existing GWAS and clinical outcome data on 7047 donor/recipient pairs available from two previous studies to augment our cohort with an estimated 2830 pairs with grade II-IV and 4244 pairs with grade 0-I GVHD, and investigate an association between the severity of acute GVHD and genotypes in donors or recipients, or mismatch of certain genotypes (other than HLA). In the second aim, we will validate the association between high risk genotypes detected in the aim 1 and severe GVHD in 1,750 donors and 1,750 recipients of allo-HSCT, and perform the joint analysis with the discovery samples (an estimated 4730 pairs with grade II-IV vs 7094 pairs with Grade 0-I GVHD). We will determine the frequency of 5000 single nucleotide polymorphisms (SNP), that GWAS in the aim 1 showed to be associated with severe GVHD and correlate it to the severity of GVHD. In the third aim, we will investigate the functional effect of SNPs detected and validated to be significantly correlated with the severity of acute GVHD in mixed lymphocyte reaction (MLR), as an Ex vivo surrogate for an alloimmune response.